It also depends on what markets we're talking about.
Consumers? Gamers and the occasional digital photo takers and websurfers? Why should they choose anything other than Windows XP Home? It's quite cheap (comes with the computer) and you can keep it up to date with Windows Update. Also, you can get about every game you like to run on that OS. Digicams come with drivers for that OS. It's perfect.
Businesses? Suddenly it's not the granny that has to install the OS. Suddenly you have an IT Admin to do it or even a whole department. Price matters here, and those persons _should_ be able to put together quite a good linux box for every employee. All the fancy stuff Windows XP and Mac OS X do don't really matter there. This is a chance linux has, and RedHat, IBM, SuSE and others are buying into that. And they might succeed.
And as different as those two are, there's a whole lot inbetween plus totally different markets. For example graphics design offices. There's a type of person I'd call the graphics-computer-geek/half-illiterate. Or something like that. And I'm one of them. I would _never_ accept that we pay a sys-admin for setting up our Macs. We're doing that on our own, because it's simple enough. We are just enough geeky to be able to do our own support of machines that go mad (which they rarely do). And that's why we're so keen to stay Mac forever. Because Apple has been providing hardware and software that work very well with our types of computer-people.
Linux isn't cutting into Apple here. Wintel does it a bit (with price and processor speed), but not really.
It's the other way round. Linux and Mac (and Sun and IBM and whatever), i.e. UN*X is cutting into Windows. On the server side. On the business side. In education. Meanwhile, Microsoft is losing its image. They're doing all they can to prevent that, but all the bugs and security holes and the fact that there just _are_ so many viruses for MS Windows variants work against them.
I applaud, when linux grows. And I applaud, when Apple grows. And I sincerely hope that we'll see a world where Microsoft holds a 60% share of the desktop computer market, Linux a 20% and Apple another 20%. Interoperability would then become an awesome business opportunity, too. And I'd become a system administrator for SMBs again...